In the past, the filter materials employed in filters to be used on the primary side of fuel pumps situated in vehicle fuel tanks (hereunder also referred to as “suction filters”) have been ones with fabric meshes, spunbond nonwoven fabrics or melt blowing nonwoven fabrics, and it has been desired for such filter materials to exhibit excellent capturing performance for particles of around 5 to 50 μm, and preferably excellent capturing performance for particles of 10 to 30 μm.
Examples of such filter materials include the filter material disclosed in the following patent literature no. 1 (PTL 1), in which a spunbond filtration medium (spunbond nonwoven fabric) or melt blow molded filtration medium (melt blown nonwoven fabric) is layered in an integral manner to form a coarse structure in the interior of the filter material, such that after removing relatively large solids with the spunbond layer, the finer solids are removed by the melt blown layer.
Also, the following PTL 2 discloses the use of a filter layer having two or more synthetic filament nonwoven fabrics layered on the inner layer of an extrusion mesh.
For removal of even finer particles, the following PTL 3 discloses the use of a filter layer having layered a synthetic filament nonwoven fabric made by an electrospinning method, instead of the aforementioned melt blown nonwoven fabric.
However, a nonwoven fabric made by a conventional spunbond method, melt blowing method or electrospinning method does not necessarily have a uniform fiber arrangement when viewed within a given small area, and it therefore lacks homogeneity of spacing between fibers, while also having a large variation in the properties relating to filter performance, such as basis weight, fiber diameter and air permeability. Because such variation manifests as variation in the performance of the filter material such as capturing performance and filter life, it is difficult to maintain stable filter performance when the filtration area is a small area of about 50-500 cm2, and therefore such materials have been considered unsuitable as filter materials to be used as suction filters.
In addition, when a synthetic filament nonwoven fabric such as a spunbond nonwoven fabric is used as the filter material, there have been problems in terms of capture efficiency for fine particles. In other words, it is difficult to obtain fiber diameters of 10 μm or smaller for fibers molded by spunbond methods, and in order to obtain the desired particle capturing performance, it is necessary to carry out a subsequent step such as surface smoothing after formation of the spunbond nonwoven fabric. In this case, high capture efficiency is exhibited similar to a synthetic filament nonwoven fabric such as a melt blown nonwoven fabric or electrospinning nonwoven fabric, but the manner of capturing is surface filtration, and a problem occurs in that the filter medium surface becomes obstructed prematurely by the particles, resulting in a short filter life.
In order to solve this problem, the following PTL 4 discloses a spun lace nonwoven fabric employing a thin woven fabric in the intermediate layer as a filter material for a suction filter.
This spun lace nonwoven fabric increases the amount of interior capturing of particles compared to a spunbond nonwoven fabric or melt blown nonwoven fabric, and therefore a relative life extension effect is obtained. However, the woven fabric section used as the intermediate layer in the spun lace nonwoven fabric does not allow passage of fluids, and therefore the woven fabric section does not function as a filter material. Consequently, the interior capturing effect for particles is not sufficiently exhibited by the spun lace nonwoven fabric.